1. Field of the Invention
This invention relates to a controller for a machine tool which executes tapping by using a rotary main shaft to which a tap is attached and a feed shaft which feeds the main shaft.
2. Background Art
In a numerically controlled machine tool, attempts have been made to increase productivity by shortening the machining time by increasing the rate of acceleration/deceleration or by increasing a maximum speed of the motors that drive the feed shaft and the main shaft of the machine tool. However, when carrying out control in the above manner, a large electric current flows into the device for driving the shafts, comprising motors and amplifiers. In addition, the motors are accelerated and decelerated several times per unit of time and, as a result, the driving device generates heat. According to the prior art, if the temperature of the driving device rises to a predetermined temperature due to heat that is generated, the machine tool is halted and an alarm occurs, and thus is prevented from being damaged by heat.
However, if the machine tool is halted during the machining operation, the machining efficiency decreases and the machining is often defective. Further, when the machine tool is operated unattended, the machine tool must be restored by an operator; i.e., the machine tool may remain halted over extended periods of time. To avoid such inconveniences, acceleration has been set by maintaining a margin within a range of a maximum frequency of acceleration and deceleration per a predetermined unit time thereby to prevent the driving device from being overheated.
However, a rise in temperature of the driving device varies depending upon the weight and material of the workpiece, machining load and ambient temperature, and the driving device may be overheated even within a maximum number of accelerations and decelerations. Overheating may also similarly result when the machining is conducted in excess of the maximum number of accelerations and decelerations. Therefore, Japanese Unexamined Patent Publication (Kokai) No 2003-5836 and Japanese Unexamined Patent Publication (Kokai) No 9-179623 disclose devices or methods capable of continuing the operation of the driving device while preventing overheating.
Japanese Unexamined Patent Publication (Kokai) No 2003-5836 discloses controlling the output of the driving unit by limiting the acceleration if the temperature of the driving unit exceeds a predetermined temperature. Japanese Unexamined Patent Publication (Kokai) No 9-179623 discloses a method of controlling an acceleration/deceleration time constant of a feed shaft to a suitable value that will not cause overheating based on the temperature of the driving means and the frequency of acceleration and deceleration.
According to Japanese Unexamined Patent Publication (Kokai) No 2003-5836 and Japanese Unexamined Patent Publication (Kokai) No 9-179623, the acceleration is varied without varying the instructed feed speed of the driving device. Therefore, according to Japanese Unexamined Patent Publication (Kokai) No 2003-5836 and Japanese Unexamined Patent Publication (Kokai) No 9-179623, the machining can be executed without increasing the machining time. However, the acceleration varies irrespective of the content of machining since the acceleration is changed depending upon the temperature of the driving unit and the frequency of acceleration and deceleration.
Further, at the machining center, the heat generated at the time of acceleration and deceleration is, generally, larger in the main shaft than in the feed shaft. For example, the tapping, is executed by repeating the acceleration and deceleration of the main shaft, and heat generates in large amounts in the main shaft if the frequency of machining is large. Further, the tapping is conducted by bringing the main shaft in synchronism with the feed shaft necessitating acceleration and deceleration in most of the machining operation. Therefore, an increase in the acceleration of the main shaft is accompanied by not only an increase in the amount of heat that is generated but also an increased error in the synchronism relative to the feed shaft.
Further, a machining tool is often broken when executing the tapping of a small diameter. Further, if the acceleration is set to meet the tapping that requires machining precision or to meet the tapping of a small diameter, then the machining time increases and the machining efficiency may decrease.
The present invention was accomplished in view of the above circumstances, and has an object of providing a controller for a machine tool capable of preventing overheating and realizing optimum machining precision while preventing breakage of the tools.